Spot welding has been commonly employed as the welding used in the construction of various thin plate metallic products. Spot welding is a method of welding that interposes a welded region of overlapped metal plates from above and below by electrodes having leading ends molded into a predetermined shape, at a predetermined pressure, and flows a predetermined current between both electrodes for a predetermined time, thereby locally heating the welded region and welding.
In addition, the surface of the welded region welded by way of spot welding is indented compared to outside of the welded part due to the pressure. This concaved part is referred to as an indentation, and the dimension of the concaved part is referred to as indentation diameter.
In addition, inside of the welded region is formed by a nugget (weld) and a crimp at the periphery thereof. The nugget is a portion at which the metal temporary melted and solidified. On the other hand, the crimp is a portion pressure bonded at the surfaces of the metal. The dimension of the nugget is referred to as nugget diameter, the nugget and crimp are collectively referred to as a joint, and the dimension of the joint is referred to as joint diameter. It should be noted that the joint is actually the joined portion.
In addition, since the overlapped metal plates are welded at a point (spot) in spot welding, it is necessary to test whether or not the weld strength is sufficient.
As a method of performing measurement of the weld strength non-destructively, a method of estimating the weld strength by measuring the nugget diameter is effective (for example, refer to Patent Document 1). Conventionally, as a method of measuring the nugget diameter, a method has been known that applies to the welded region a magnetic field generated by coils through which current flows, and then obtains the nugget diameter from the change in inductance of the coils generated as a result thereof. With the conventional method, since there is a property whereby the magnetic permeability (μ) changes between the nugget and portions other than the nugget, the change in magnetic permeability (μ) is detected as the change in inductance using this property, whereby the nugget diameter is obtained.
Patent Document 1: Japanese Patent Publication No. 3098193